Abstract: In recent years, the universal type highvoltage cascade frequency converter became key device for energy conservation due to its structural advantages. The output current of the converter is the key variable for overload protection, overcurrent protection and V/F control compensation, so it is very necessary to measure the realtime current effective value accurately. The fast true RMS method using 1/6 period wave was used to calculate the effective value of the current signal. Then the fitting method was used to obtain the practical output current of the converter. The algorithm is one of true RMS algorithms and suitable for detection of current signal containing harmonics; and in this method, the repeated mean value filtering necessary for other methods is avoided, so the calculation time is reduced. By this method, the control system of the converter can provide more accurate reference for the overload protection circuit, and the misoperation rate of the system is decreased, which provides possibility to set more accurate safety margin for the equipment and to ensure safe operation of the equipment.

Key words: universal frequency converter, true effective value (RMS), mean value filtering, overload protection, detection method